1. Field of the Invention
The present invention relates to a conductive paste used for formation of thick film electrodes on terminals of electronic parts or substrates. More particularly, the present invention relates to a conductive paste for providing thick film electrodes of which the bonding strength is prevented from lowering by plating.
2. Description of the Prior Art
Recently, surface mounting has become the leading mainstream process for mounting electronic parts on substrates and the requirement for improvement in solderability of thick film electrodes of the parts has become strict. Such electrodes are usually produced by applying a conductive paste on substrates or terminals of electronic parts and then firing the substrates to remove organic components contained in the paste. The conductive paste used for this purpose is usually composed of a conductive powder (e.g., Ag) dispersed in an organic vehicle together with a glass frit containing lead oxide with a relatively low softening point as the addition of such a glass frit makes it possible to produce electrodes with superior adhesion strength.
On the other hand, the thick film electrodes are occasionally covered with a tin plating or solder plating to improve solderability of the electrodes formed on substrates or terminals of electronic parts. However, plating lowers the properties of thick film electrodes. For example, the electrodes made from a conductive paste contains 2 wt % of a glass frit consisting of 10.5 to 53.6 wt % of PbO, 10.7 to 31.9 wt % of SiO.sub.2, 32.7 to 46.4 wt % of B.sub.2 O.sub.3, 0 to 9.6 wt % of ZrO.sub.2, and 0 to 42.7 wt % of ZnO, have adhesion strength of about 15 kgf before plating, but after plating the adhesion strength is lowered to 2.0 kgf or below and the electrodes are peeled off from the substrates under certain circumstances.
To solve such problems, it has been proposed to use a lead borosilicate glass fit containing one or more alkaline metals. However, the glass phases containing lead oxide are eroded easily by an electroless plating solution, resulting in failure in effective improvement in adhesion strength of electrodes.